Transcript Confinement- and Template-Assisted Self

Solar Cells:
Energy for the Future
Basic Solar Cell Design
DOE - Solar Energy Technologies Program
National Renewable Energy Laboratory
Page2
Measures of Efficiency
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Short Circuit Current
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Open Circuit Voltage
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Green, Martin A. “Solar Cells – Operation Principles,
Technology and System Applications”
500~700mV
Fill Factor
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40~50mA/cm2
“Illumination” current
“Square area”
0.7-0.85
Efficiency
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Production: 10-15%
Laboratory: 20-25%
Page3
Efficiency Losses
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Light reflection
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Silicon
Electrical contact
coverage
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Cell thickness
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Lower collection
probability away from
depletion region
Material dependent
Material resistances
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Wavelength of Light
Both bulk and contact
Temperature
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Metal and semiconductor
dependence
Recombination
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Defect states
Page4
Silicon – Various Types
DOE - Solar Energy Technologies Program
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Single-crystal silicon
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Polycrystalline silicon
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Czochralski
Float-zone
Ribbon
Amorphous silicon
Evergreen Solar Technology
Page5
Materials -Silcon
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Silicon
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Indirect bandgap Eg = 1.142eV
Low absorptivity
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Photon travels farther before
absorbed
>100µm thick
Photon + Phonon absorption
processes (indirect)
Recombination
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Dominated by defects
 Impurities and surface states
Green, Martin A. “Solar Cells – Operation Principles,
Technology and System Applications”
Page6
Materials-Silicon
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Silicon (continued)
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Doping (~1016 cm-3)
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P-type: Boron
 Trace amounts in Cz growth process
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N-type: Phosphorus
 POCl3 + oxygen gas stream in heated furnace to oxidize Si
 Diffusion of P from oxide into Si
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Contacts
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Vacuum evaporation
Three layers
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Ti for good Si adherence
Ag for high conductivity
Pd barrier layer inbetween
Sintering at high T (500-600°C) for low resistance and high adherence
Page7
Materials- Silicon
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Contacts (continued)
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Antireflective Coating
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Back is completely covered
Metal grid on front
Vacuum evaporation
Various oxides of Si, Al, Ti, Ta…
Encapsulation
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Structural back for support and moisture resistance
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Al, Steel, Glass
Transparent front for light transmission
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Glass
Page8
Typical Silicon Cell Design
Single and Polycrystalline
Silicon
The Solarserver Forum
Amorphous Silicon
DOE - Solar Energy Technologies Program
Page9
Improving Silicon Cell Design (I)
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Textured top surface
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Surface passivation
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Selective etching to couple light
into cell
SiOx or SiNX
Restores bonding state of dangling
surface Si bonds
Back Surface Field
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Low recombination velocity
interface
Screen print Al and fire to alloy
Green, Martin A. “Solar Cells – Operation Principles,
Technology and System Applications”
Page10
Improving Silicon Cell Design (II)
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Layer thickness
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Green, Martin A. “Solar Cells – Operation Principles,
Technology and System Applications”

Thinner = lower light
absorption
Carrier diffusion length and
surface passivation
important
If high recombination, then
want thinner
Contact placement
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Both on back: ~25%
efficiency
Handbook of Photovoltaic Science and Engineering
Page11
Silicon Cell Efficiency
Material
Laboratory Efficiency [%]
Production Efficiency [%]
Single crystal silicon
~ 24
14-17
Polycrystalline silicon
~ 18
13-15
Amorphous silicon
~ 13
5-7
Wikipedia.org
Page12
Costs
Handbook of Photovoltaic Science and Engineering
Page13
Structure Comparison
Single Crystalline
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Highest efficiency
Many processing
techniques
Purity = Process dependent
Expensive
Circular cells
Huge market
High waste (ingot)
Excellent electrical
properties
Polycrystalline
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Cheaper than single crystalline
Less efficient
More easily formed into squares
High waste
Page14
Advantages/Disadvantages of Silicon
ADVANTAGES
Second most abundant
element in the crust
 Well-developed
processing techniques
 Huge market for
crystalline Si
 Highest efficiency
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DISADVANTAGES
Need thick layer
(crystalline)
 Brittle
 Limited substrates
 Expensive single
crystals
 Some processing
wasteful
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Page15
Other Inorganic Solar Cells
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Amorphous Si-based Solar Cells
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Cu(InGa)Se2 Solar Cells
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Cadmium Telluride Solar Cells
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GaAs
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InN Solar Cells
Page16
Motivation for Other Materials
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Graph of Semi-conductor band
gap vs. Efficiency
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A band gap of ~1.4eV matches
the photon energies where the
sun’s spectral intensity is
strongest
GaAs is an example of a material
with an optimal band gap
Silicon Band Gap is
1.1 eV,
not optimal
This explains why there is a
maximum in efficiency for single
layer devices
Green, Martin A. “Solar Cells – Operation Principles,
Technology and System Applications”
Page17
Amorphous Si Solar Cells
Amorphous Silicon Semiconductor
 First made 1974
 Plasma deposited
 Doping
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Hydrogen helps properties
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p-type: B2H6
n-type: PH3
hydrogenated amorphous silicon (a-Si:H)
Alloying changes the band gap
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Ge, C, O, or N
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Ge used for bilayer devices
Page18
a-Si:H: Photodiode Design
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Photodiode: three layers
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Built-in E-Field
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(typical example)
20 nm p-type layer
Few hundred nm intrinsic layer
20 nm n-type layer
~ 104 V/cm
Voc
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Varies with band gap
Band gap varies with alloying
Handbook of Photovoltaic Science and Engineering:
Depiction of an a-Si:H photodiode
Page19
a-Si:H: Photodiode Design
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Direction of incoming light
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Photons reach p-type first
Asymmetry in the drift of holes
and electrons
Power drop if lighted from the ntype side
Width of Intrinsic layer
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Thicker cells do not absorb much
more light
Best thickness around 300nm
(power saturates)
Handbook of Photovoltaic Science and Engineering:
Computer calculation of Power vs. Intrinsic Layer Thickness for different absorption coefficients. Solid symbols indicate
illumination through the p-layer. Open Symbols indicate illumination through the n-layer
Page20
a-Si:H: Cell Design
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Two types of cell design
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Handbook of Photovoltaic Science and Engineering:
Design of the cell
Superstrate (left): better for applications in which the glass substrate can be an
architectural element
Substrate (right): Substrate can be flexible Stainless Steel
Substrate affects the properties of the first photodiode layer deposited
Page21
Advantages of a-Si:H
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Technology simple and
inexpensive compared to
crystalline technology
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Absorbs more light: need less
material than c-Si
Better high temperature
stability than c-Si
Band gap:
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Still need to lower costs
variable, 1.4-1.8 eV
Efficiency ~15%
Handbook of Photovoltaic Science and Engineering:
IV curves for amorphous silicon solar cells at two different times
Page22
Further Advantages
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High light absorption
Very little needed (~1/100th)
Produced at lower T
Many substrates
Low cost
Disadvantages
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Must be hydrogenated
Low efficiency
Poor electrical properties
Page23
Advantages of Other Materials
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Cu(InGe)Se2 (CIGS)
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Thin film: easy fabrication, low cost
Band gap: variable, 1.0-1.2 eV
High efficiency – up to 18.8%
High radiation resistance
Can take large variations in composition without appreciably affecting the
optical properties
Cadmium Telluride (CdTe)
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Also Thin Film
Band gap in optimal range: 1.5eV
Efficiencies of about 7%
Page24
Advantages of Other Materials
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GaAs
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Band gap in the optimal range: 1.4 eV
Efficiencies of >20% shown (1982)
InN
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Optical band gap is also a good match to the sun’s spectrum: can tune
the band gap
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This means that multiple layers can be used to absorb different
wavelengths and the crystal structures won’t mismatch
Band gap: 0.7 eV
Large heat capacity, resistant to radiation
many defects but this does not affect light emitting diodes of the same
material
Page25
Dye Sensitized Solar Cell (Grätzel Cell)
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Overall power conversion
efficiency of 10.4% has been
attained
(US National Renewable Energy Laboratory)
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General Structure:
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Glass
Transparent Conductor (ITO)
Semiconducting Oxide (TiO2)
Dye
Electrolyte
Cathode (Pt)
Glass
M. Grätzel, “Dye Sensitized Solar Cells,” Journal of Photochemistry and Photobiology C: Photochemistry Reviews, 4, 145–153 (2003)
Page26
Components (I)
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Mesoporous oxide films:
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Network of tiny crystals
measuring a few nanometers
across.
Can be TiO2, ZnO, SnO2, Nb2O5,
CdSe
Exceptional stability against
photo-corrosion
Large band gap (>3eV)
= transparency for large part of
spectrum
SEM of the surface of a mesoporous anatase film
prepared from a hydrothermally processed TiO2
colloid.
M. Grätzel, “Photoelectrochemical cells,” Nature, 414, 338 (2001).
Page27
Components (II): The dye
Dye absorbs light and generates current in the entire visible spectrum
M. Grätzel, “Dye Sensitized Solar Cells,” Journal of Photochemistry and Photobiology C: Photochemistry Reviews, 4, 145–153 (2003)
Page28
Components (III)
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Mesoscopic pores
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filled with a semiconducting or a conducting medium (such
as a p-type semiconductor, a polymer, a hole transmitter or
an electrolyte)
Traditional electrolyte material consists of iodide (I-) and
triiodide (I3-) as a redox couple.
M. Grätzel, “Photoelectrochemical cells,” Nature, 414, 338 (2001).
Page29
DSSC: Operation
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Mesoporous dye-sensitized TiO2,
receives electrons from the
photo-excited dye
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Oxidized dye in turn oxidizes
the mediator in electrolyte
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Mediator is regenerated by
reduction at the cathode.
M. Grätzel, “Photoelectrochemical cells,” Nature, 414, 338 (2001).
Page30
DSSC: Degradation
1.
2.
3.
4.
5.
Photo-chemical or chemical degradation of the dye (e.g.
desorption of the dye, or replacement of ligands by electrolyte
species or residual water molecules)
Direct band-gap excitation of TiO2 (holes in the TiO2 valence band
act as strong oxidants)
Photo-oxidation of the electrolyte solvent, release of protons from
the solvent (change in pH)
Dissolution of Pt from the counter-electrode in contact with
electrolyte
Adsorption of decomposition products onto the TiO2 surface.
J. Halme, “Dye-sensitized nanostructured and organic photovoltaic cells: technical review and preliminary tests,”
Helsinki University of Technology, Masters Thesis (2002).
Page31
DSSC: Benefits
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Relatively cheap to fabricate
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the expensive and energy-intensive high-temperature and highvacuum processes needed for the traditional devices can be avoided
Can be used on flexible substrates
Can be shaped or tinted to suit domestic devices or
architectural or decorative applications.
Stable even under light soaking for more than 10,000 h (with
certain conditions/materials that are less efficient).
M. Grätzel, “Photoelectrochemical cells,” Nature, 414, 338 (2001).
Page32
DSSC: Drawbacks
Efficiencies not yet commercially competitive with
Si-based alternatives.
 Degradation still an issue
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EC Cell cycles important to operation
Encapsulation necessary
High temperature stability a problem
Production only at small scale
Page33
DSSC: Costs
$0.40/Wp at 5% module efficiency (Zweibel 1999)
J. Halme, “Dye-sensitized nanostructured and organic photovoltaic cells: technical review and preliminary tests,”
Helsinki University of Technology, Masters Thesis (2002).
Page34
Organic Heterojunction Solar Cells
Bilayer
P.Peumans, S.Uchida, S.R.Forrest. Nature, 425, 158 (2003).
Bulk Heterojunction
Efficiency of 3.5% has been achieved
Page35
Summary of PV & PEC cells
M. Grätzel, “Photoelectrochemical cells,” Nature, 414, 338 (2001).
Page36
Comparison
SPIE Magazine of Photonics Applications and Technologies
Page37
2
Emissions
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PV will be responsible
for the displacement of
millions of metric tons
of CO2 per year, even
under the most modest
estimates
V Fthenakis, S Morris PREDICTIONS OF FUTURE PV
CAPACITY AND CO2 EMISSIONS' REDUCTION IN THE US.
2003
Page38
Environmental Impact – Other Pollutants
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According to economic models, PV will result in the
reduction of NOx, soot, and SO2
V Fthenakis, S Morris
Page39